The objectives of this study are to examine the effect of certain B-adrenergic anti-labor drugs on uterine and umbilical blood flows, maternal and fetal heart rates, arterial pressures, arterial blood gases and plasma glucose concentrations, and to evaluate the effect of these agents on maintenance or restoration of fetal well-being. Pregnant Dorset sheep are operated on between 125-145 days gestation (term equals 145 days). Electromagnetic flow probes and catheters are placed on/in pertinent maternal and fetal vessels and chronic preparation is obtained. After a minimum of 48 hours to allow for recovery and reestablishment of maternal and fetal homeostasis, the test drug, ritodrine HCl, Salbutamol, or fenoterol (Thll65a), is infused into the peripheral maternal vein. It is administered at either a constant dose for 2 hours or a progressively increasing dose with step increments every 15 minutes. Physiologic measurements are then obtained utilizing the surgically placed flow probes and catheters.